1. Field of the Invention
The present invention relates to a power tool such as an electric driver and an electric driver drill in which a rotational speed of a spindle can be changed in three levels by utilizing a planetary gear mechanism.
2. Description of the Related Art
An electric driver drill is known in which the rotational speed of a spindle can be changed in three levels, or a predetermined first speed (hereinafter refers to as “low speed”), a second speed fester than the first speed (hereinafter refers to as “medium speed”) and a third speed faster than the second speed (hereinafter refers to as “high speed”). In this electric driver drill, a plurality of planetary gear mechanisms are disposed in series in the axial direction between an output shaft of a motor that is housed within a housing and a spindle that is disposed in a tip end region (front end region) of the housing. Further, die planetary gear mechanisms are constructed such that a plurality of planetary gears revolve on a sun gear while rotating within an internal gear. Such a driver drill is disclosed, for example, in Japanese laid-open patent publication No. 2004-237422.
The known driver drill has a speed changing part that can be linearly slid in the axial direction by a user from the outside of the housing. When the speed changing part is slid such that one of the two front and rear internal gears in the axial direction is locked to the housing and thus prevented from rotation, the spindle rotates at low speed. When the speed changing part is slid such that the other of the two internal gears is locked to the housing and thus prevented from rotation, the spindle rotates at medium speed. When the speed changing part is slid such that either one of the two internal gears is connected to a carrier while both of the internal gears arc allowed to rotate, the spindle rotates at high speed.
In order to realize reduction in size of the driver drill, it is preferable that the outside diameter of the internal gear is minimized. In other words, in order to meet such a demand for reduction in size of the driver drill, a constraint is imposed that the outside diameter of the internal gear cannot be increased. Under such a constraint, it is difficult to widen the difference between the speed reduction ratio of one planetary gear mechanism and the speed reduction ratio of the other planetary gear mechanism. As a result, it is rendered difficult to widen the difference m speed of the spindle between during rotation at low speed in which either one of the internal gears is locked to the housing and prevented from rotating, and during rotation at medium speed in which the other internal gear is locked to the housing and prevented from rotating. Therefore, in actual screw-tightening operation by using a driver drill, the frequency of shifting from low speed to medium speed or from medium speed to low speed is extremely lower than the frequency of shifting from low speed to high speed or from medium speed to high speed.
However, in the known driver drill, when the speed changing part is operated to change the rotational speed of the spindle, the speed changing part is shifted from low speed to medium speed and then to high speed, and vice versa. Therefore, in order to change from low speed to high speed or from high speed to low speed, the rotational speed of the spindle must be once changed to medium speed. Therefore, such an operation is troublesome, and further improvement is desired in this respect.